In manufacturing reinforced plastic pipe by the wet winding process, a number of rovings or filament bundles are drawn through liquid resin, collected into a band, and wound onto a rotating mandrel. The winding path is usually a simple helix at an angle of approximately 55.degree. to the mandrel axis of rotation. A uniformly reinforced pipe wall is formed by depositing the band of rovings wound in such a helix, first to the right along the mandrel and then to the left.
The economics of the winding process tend to improve as the number of rovings in the band is increased. However, if the added rovings are used to make the band thicker, several problems can be concountered, especially in small pipe with relatively thin walls. For example, it is more difficult to make a pipe wall of uniform thickness and good appearance when a thick, relatively narrow band is used. Also, the structural discontinuities created where thick bands cross each other within the wall tend to reduce the cyclic pressure resistance of the pipe. For these and other reasons, excessively thick bands tend to produce an inferior product. It is therefore preferable in most cases to increase the width of the band rather than its thickness to accommodate more rovings.
A number of methods for saturating rovings with liquid resin and collecting them into a band are in common use. Most of these methods fall into one of two categories: a "wet comb" method and a "dry comb" method. In both of these categoreis the same basic technique is used to form the band. This technique consists of using a comb or equivalent structure placed transverse to the path of the rovings to space the rovings uniformly along a straight line, and a concavely curved bar over which the rovings are drawn onto the mandrel. It is contact with the concave side of the curved bar that gathers the rovings into a uniform band. In the dry comb version of this technique, the rovings pass through the comb first, then through the resin bath and finally over the curved bar onto the mandrel. In the wet comb variant, the rovings are drawn through the resin bath first and subsequently through the comb and over the bar. When the wet comb method is used, the comb can be as close to the curved bar as desired since the resin bath is placed ahead of the comb. With the comb close to the bar, the rovings can be made to converge sharply to the curved bar and the bar can be strongly curved. When the spacing between the bar and the comb and the curvature of the bar are skillfully combined, the band can be tightly controlled. For example, the width of the band will change very little during carriage reversals and the band will quickly settle into its new position on the curved bar once the reversal is completed. Therefore, deviations from the desired helical winding angle and band width caused by carriage reversals are limited to very short sections at each end of the pipe.
A disadvantage of the wet comb method is the tendency of the comb itself to produce tangles. This characteristic is a result of small bundles of broken fibers within individual rovings. While rovings containing such flaws pass quite reliably through a comb structure when they are dry, they frequently tangle once they are wet. When the number of rovings used is large, and the product rate of the winding machine is high, work stoppages due to roving tangles can be costly.
Hirtzer in U.S. Pat. No. 3,492,187 describes a wet roving filament winding mechanism. Hirtzer's mechanism includes a resin pot, three combs, and delivery bar sequentially positioned with the added feature of pivoting the comb and delivery bar about a vertical axis to further improve control over the band when the winding helix reverses. However, even with this added feature, tangling and breaking of fibers can still occur at the wet combs. If Hirtzer's three comb mechanism is used with a large number of rovings, say, 50 to 100, excessive tangling of fibers would be inevitable.
Compared to the wet comb method, the dry comb system is highly reliable. Its primary disadvantage is the relatively poor band control it affords. This deficiency is due to the larger distance between the comb and the bar that is necessary to make room for the resin bath. Because of the extra distance, the rovings cannot be made to converge as sharply from the comb into the bar and so the bar cannot be strongly curved. If a wide band is desired, the curvature of the bar must be reduced still more. As a result, the band usually widens during carriage reversals and it must move a considerable distance along the gently curved bar to reach its new equilibrium position. But most important, the band is slow to reach its equilibrium width and location on the bar after a carriage reversal so a significant amount of material is wound at an incorrect band width and helix angle at each end of the mandrel. It is necessary to cut off and discard some of the incorrect windings at each end of the pipe if a high quality product is required. This extra trim loss can reach 5% or more of the total pipe length so it is a significant consideration.
In order to utilize the dry comb process to obtain reliability without sacrificing control when the helix reverses, one solution suggested is a tightly curved delivery bar such as a toroidal delivery ring as taught by Young in U.S. Pat. No. 3,025,205. However, the highly curved bar permits only relatively narrow bands to be wound around the mandrel, thereby adversely affecting the economics of the process.
Another method suggested for good control when the helix reverses is described by Hoff in U.S. Pat. No. 3,499,815. Hoff teaches the use of a horizontally oriented bar which pivots on a vertical axis for delivery of rovings to a horizontal mandrel. With this arrangement the width and thickness of the band are relatively unstable because the rovings tend to slide across the surface of the bar. In addition, the desired helix angle is not restored quickly because of a large distance between the comb and the mandrel.
Therefore, in the wet winding process, there is need for a reliable method and apparatus for forming a wide band of uniformly spaced rovings and winding them on a mandrel in a manner that limits deviations from the desired winding pattern to very short lengths at each end of the mandrel.